The present invention relates to a process for the manufacture of sodium bisulfite. More specifically, the invention relates to an improved process for manufacturing sodium bisulfite from a waste stream generated while producing sodium sulfite.
Sodium sulfite, Na2SO3, can be prepared by reacting sulfur dioxide gas, SO2, in a solution containing a sodium source, e.g. sodium hydroxide (NaOH) or sodium carbonate (Na2CO3). A typical manufacturing process is described in U.S. Pat. No. 4,003,985 to Hoffman et al. Sodium bisulfite, NaHSO3, can be prepared by reacting sulfur dioxide gas, SO2, in a solution containing alkaline hydroxide, e.g. sodium hydroxide (NaOH) or sodium carbonate (Na2CO3). Also, under certain conditions, this reaction can produce sodium sulfite, Na2SO3. Reacting a solution of sodium sulfite with additional sulfur dioxide gas can produce sodium bisulfite.
Sodium bisulfite has a number of commercial uses including as a disinfectant, a bleaching agent, a food preservative, and as a mild reducing agent for removal of small amounts of chlorine, bromine, iodine, hypochlorite salts and oxygen (i.e. an oxygen scavenger agent).
Some commercial techniques for producing sodium sulfite use an evaporative crystallizer requiring a purge stream to remove impurities that can accumulate and contaminate a sodium sulfite product. The discharged purge stream normally consists of saturated sodium sulfite in water with sodium chloride, other water soluble impurities, and insoluble matter. The purge steam is typically discarded. The discarded sodium sulfite from the purge stream represents an inefficient loss of potentially recoverable and valuable resource material.